Process for enhanced urea production

ABSTRACT

A process for the production of urea comprising the steps of reacting carbon dioxide and ammonia to form ammonium carbamate and subsequent decomposition of ammonium carbamate to form a reaction mixture comprising urea and water, wherein the reaction mixture is contacted with one side of a semi-permeable membrane, and a drying fluid capable of removing water and possibly other reaction products(s) from the reaction mixture is contacted with the other side of the semi-permeable membrane.

This invention relates to an improved process for enhanced conversionand/or enhanced rates of conversion in the production of urea byreaction of carbon dioxide and ammonia.

BACKGROUND OF THE INVENTION

As is well known, carbon dioxide and ammonia can be caused to react witheach other to form ammonium carbamate. It is also known that ammoniumcarbamate can be converted into urea and water. Moreover, urea can beproduced directly from carbon dioxide and ammonia by causing thereaction to take place at a suitable temperature and pressure and for asufficient period of time to allow the initially formed ammoniumcarbamate to be converted to urea. Typically, temperatures greater than150° C. and pressures greater than 10MPa are used. This direct processis the basis for most commercial synthesis of urea at the present time.It is, however, also known that at any commercially practical suitabletemperature and pressure, the percentage conversion, (which is theproportion of carbon dioxide fed to the process which is converted tourea, expressed as a percentage) is limited. It is further known thatthis limit is essentially due to an equilibrium being established as aresult of the reverse reaction between water and urea. According to themost recent correlation in the literature (D. M. Gorlovskii and V. I.Kucheryavyi, Zhurnal Prikladnoi Khimii 53.11, 2548-2551 November 1980)the maximum possible conversion to urea at equilibrium is close to 86percent, however the highest experimentally observed conversion to ureais close to 84 percent. Several of the most recent urea processes havebeen described by I. Mavrovic and A. R. Shirley in an article(Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition,Published by John Wiley & Sons New York (1983) Vo. 23, p. 548-575),which indicates that in the present state of art, the highest conversionof carbamate to urea in a single pass is achieved by the UTIHeat-Recycle Process, with a reported 72-74 percent conversion per pass.In U.S. Pat. No. 3,236,888 (Wentworth) the only example cited gives theconversion to urea based on carbon dioxide introduced to the reactor asabout 76 percent per pass. E. Guccione, (Chemical Engineering, Sept. 26,1966 p. 96-98), in discussing this same process claims that the highertemperatures (380° to 450° F.) permit high conversion of CO₂ to urea of80 to 85 percent.

It is also known from the literature (see Krase and Gaddy, JournalAmerican Chemical Society 52, 3088-3093 (1930)) that attempts have beenmade to remove water produced by the above described process by usingdehydrating agents, in either the gas or liquid phase, to increase theconversion of carbamate to urea.

In Ruf et al., (Swiss Chem 6 (1984) Nr. 9, 129-141 and Swiss Chem 8(1986) Nr. 10a, 18-25) there is a theoretical description relating tosimulation of a urea plant and enhancement of the yield based on ahypothetical new technology in which a semipermeable membraneselectively removes water from urea melts. It is stated in the latterpaper that a reverse osmosis process is visualized in which the water isremoved, across the membrane which forms the wall of the reactor, bymeans of the high pressure in the reactor and which overcomes theosmotic pressure gradient.

The paper states that no membrane is known to the authors which iscapable of carrying out the process, but the future development of suchis forseeable. It is further stated that the simulation, andcomputationally demonstrated benefits and improved methods of operationtheoretically possible with the process, are not confined to theparticular membrane process, but apply to any workable water removalprocess.

SUMMARY OF THE INVENTION

In work leading to the present invention, it has now been found that itis possible to use a suitable semi-permeable membrane to remove waterand possibly other reaction product(s), including urea, from thereaction mixture derived either from ammonia and carbon dioxide or fromammonium carbamate. Accordingly, the process of the present inventionprovides an improvement in the conversion of the starting materials tourea and/or in the rate of conversion of those materials to urea.

According to the present invention, there is provided a process for theproduction of urea which comprises the steps of reacting carbon dioxideand ammonia to form ammonium carbamate and subsequent decomposition ofammonium carbamate to form a reaction mixture comprising urea and water,wherein the reaction mixture is contacted with one side of asemi-permeable membrane, and a drying fluid capable of removing waterand possibly other reaction product(s) from the reaction mixture iscontacted with the other side of the semi-permeable membrane.

The ammonium carbamate may be formed in situ by reaction of ammonia andcarbon dioxide in a synthesis/conversion enhancement zone in contactwith the semi-permeable membrane. Alternatively, the ammonium carbamatemay be formed in a separate urea reactor as a synthesis zone, and thereaction mixture then passed to a conversion enhancement zone in contactwith the membrane. In either case, an ammonium carbamate recycle mixturemay, if desired, also be admitted to the synthesis zone together withammonia and carbon dioxide in order to further improve the efficiency ofconversion. The ammonia and carbon dioxide are used in a molar ratioknown in the production of urea, typically a ratio of ammonia to carbondioxide of greater than 2.

The reaction may, for example, be carried out at a temperature in therange of 140° to 250° C., preferably 160° to 220° C., and at a pressurein the range of 15 to 50 MPa, preferably 25 to 45 MPa.

It will be understood that the semi-permeable membrane must be made of amaterial which can withstand the high temperature and pressureconditions involved in the urea synthesis process. In one embodiment ofthis process, the temperature or pressure may be adjusted so as to be atleast substantially the same on the two sides of the membrane.Alternatively, however, the temperature and/or pressure may be differenton the two sides of the membrane, provided that the membrane issufficiently strong or sufficiently supported to withstand the pressuredifferential. The membrane must also have a higher permeability for thetransport of water and possibly also urea than for the other componentsof the reaction mixture, particularly carbon dioxide and ammoniumcarbamate. Suitable materials are described in detail hereinafter.

The drying fluid used in accordance with this invention may be any fluidwhich can cause water and possibly other reaction product(s), includingurea, in the reaction mixture to preferentially migrate across themembrane. Ammonia is the presently preferred drying fluid. In itssupercritical state (above 132° C. and 12 MPa), ammonia forms a densefluid having a high affinity for water and dissolves urea. It will,however be understood that other drying fluids such as air or carbondioxide may also be used in accordance with the invention.

The present invention differs from a conventional reverse osmosisprocess (suggested in the Ruf et al papers referred to above) in the useof a drying fluid as described herein on the other side of the membraneto the reaction mixture, and may, for example, be carried out with thepressure substantially equal on both sides of the membrane.

In general terms, the present invention includes a process for theproduction of urea (carried out in either one or two stages) whereincarbon dioxide, ammonia, and/or ammonium carbamate is reacted in asynthesis zone, and if necessary the reaction mixture is transfered fromthe synthesis zone into a conversion enhancement zone, which includes:

(a) utilizing a semi-permeable membrane, which has a higher permeabilityfor the transport of water and possibly also urea, than for the othercomponents of the reaction mixture, particularly than for carbon dioxideor ammonium carbamate, in the conversion enhancement zone, with the saidsemi-permeable membrane being suitably held and supported;

(b) feeding the reaction mixture to one side of the said semi-permeablemembrane at a suitable high temperature and pressure;

(c) feeding a drying fluid such as ammonia to the other side of the saidsemi-permeable membrane at a suitable temperature and pressure;

(d) removing a portion of the enhanced reaction mixture, containing alesser percentage of carbon in the form of carbon dioxide and ammoniumcarbamate combined (and usually a higher percentage of urea) than in thesaid reaction mixture,

(e) recovering urea from the removed portion of the enhanced reactionmixture, and if desired,

(f) recovering urea from the drying fluid.

The semi-permeable membrane used in the process of the present inventioncan be made from any membrane material which has the required propertiesand is sufficiently stable under the temperature and pressure conditionsof the process. For example, such a material can be aperfluorocarboxylate membrane (such as FLEMION), a perfluorosulphonatemembrane (such as NAFION 117), a reinforced version of such a membrane(for example NAFION 423 or NAFION 324) or a combination of these typesof structure (such as NAFION 901). Alternatively, another material or acomposite material such as one combining one or more materials, forinstance polymers, with other types of materials, for example a porousceramic membrane material, may be used to achieve improved conversionand/or stability. The said semi-permeable membrane can be in anyconvenient form, such as for example in the form of film, as a sheet orin the form of hollow fibres or of tubes, as is well known to thoseskilled in the technology of membranes. (FLEMION is the registered trademark of Asahi Glass Co. Ltd., Japan; NAFION is the registered trade markof E. I. DuPont de Nemours & Co., U.S.A.)

In tests conducted to date, it has been established that the process ofthe present invention is capable of achieving a higher conversion tourea in a single pass through the process than any previously reportedurea synthesis process. In addition, the present invention enables aspeeding-up of the urea conversion process.

Further details of the present invention will now be described by way ofexample, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. depicts schematically one apparatus for carrying out the processof the invention and which utilizes an external conversion enhancementzone (External CEZ);

FIG. 2. depicts schematically another apparatus for carrying out theprocess of the invention and which utilizes an internal conversionenhancement zone (Internal CEZ);

FIG. 3. shows the glass reaction cell (GRC). referred to in Example 1.

DETAILED DESCRIPTION OF THE PROCESS OF THE INVENTION

As described above, a feature of the process is the transfer of waterand possibly other reaction product(s), including urea, from theconversion enhancement zone across a suitable semi-permeable membrane toa drying fluid to allow the forward reaction of carbamate to urea to beenhanced relative to the reverse reaction, thus increasing conversion.As previously described, the drying fluid can be any fluid which cancause water and possibly other reaction product(s) to preferentiallymigrate from the conversion enhancement zone across the membrane, forexample ammonia.

The invention may, for example, be performed in either of two ways. Theconversion enhancement zone (CEZ) can be either incorporated in thesynthesis reactor (Internal CEZ), or it may be incorporated in aseparate reactor following the synthesis reactor (External CEZ). Eitherof the two methods can be utilized in a new urea process plant. Byretrofitting, the latter method (External CEZ) will be applicable toexisting plants and the former method (Internal CEZ) may be applicableto existing plants in suitable cases depending on the existing processdesign.

The External CEZ embodiment, illustrated in the accompanying diagram ofFIG. 1, which is applicable to the general case of retrofitted existingurea plants or to new plants, comprises a vessel 11 containingsemi-permeable membrane(s) 5 located downstream of the urea reactor(synthesis zone) 3 and fed with reaction mixture 12. Simultaneously, adrying fluid 6 is fed to the drying zone 8 which is on the opposite sideof the semi-permeable membrane 5. There is a preferential transferenceof water and possibly other reaction product(s) from the fluid in theconversion enhancement zone 4 to the fluid in the drying zone 8 acrossthe semi-permeable membrane 5 thus causing further reaction of carbamateto urea in the conversion enhancement zone 4. As a further option,carbamate recycle mixture 10 may be fed additionally to the synthesiszone 3 and/or the CEZ 4. An enhanced reaction mixture 9 is removed fromthe CEZ 4, and urea is recovered from the enhanced reaction mixture 9,and optionally from the fluids 7 removed from the drying zone 8.

The Internal CEZ embodiment shown schematically in FIG. 2, is applicablein the case of new urea plants and those existing urea plants withsuitable reactors. The membrane(s) 5 is (are) incorporated into theprimary reactor 13 which includes the synthesis zone 3 and the CEZ 4. Inaccordance with common practice, carbon dioxide 1 and ammonia 2 are fedto the urea reactor for reaction first to carbamate then to urea in thesynthesis zone 3. As an option, carbamate recycle mixture 10 may also befed additionally to the synthesis zone 3. There is a preferentialtransference of water and possibly other reaction product(s) from theconversion enhancement zone (CEZ) 4 across the membrane 5 to the dryingfluid 6 fed to the drying zone 8, thus causing further reaction ofcarbamate to urea in the CEZ 4. An enhanced reaction mixture 9 isremoved from the CEZ 4, and urea is recovered from the enhanced reactionmixture 9, and optionally from the fluids 7 removed from the drying zone8.

The following example of the invention are not intended to be limitingin nature, but illustrate the improved conversion which may be obtainedin accordance with this invention.

EXAMPLE 1

(a) Apparatus:

The apparatus used in this Example is illustrated in FIG. 3. As shown, aglass reaction cell 14 (GRC) consists of two halves 15, 16 (each of 9.3cm³ volume) separated by a semi-permeable membrane of NAFION 117 (aperfluorosulphonate membrane) 17 (in the first experiment only) andteflon gaskets 18. The experiments were conducted under a pressure ofanhydrous ammonia in a stainless steel autoclave with a glass liner. Thepressure was transmitted to both halves of the GRC by glass capillaries19, 20 (each 1 mm ID.). The capillaries served to prevent loss of thereagents from the GRC during the period of the reaction.

(b) Preparation of Ammonium Carbamate:

Ammonium carbamate was prepared by passing dry carbon dioxide gas intoanhydrous liquid ammonia in a glass apparatus, removing excess ammoniaand carbon dioxide in a vacuum desiccator, and storing the dry carbamatein sealed glass ampoules prior to use.

(c) Analytical Method for Urea:

The products of the experiments were collected and analysed for ureagravimetrically by allowing most of the excess ammonia to evaporate,dissolving the products in water (100 to 300 cm³) and drying in a rotaryevaporator under vacuum at 85°±3° C. The gravimetric method was testedon mixtures of carbamate and urea over the whole range of concentrationsand found accurate to better than ±0.5%. Elemental analysis of thecarbamate and of recovered urea showed that C,H,N and O were in bothcases in the correct proportions within the accuracy of the analysis of±0.4%.

(d) Reaction With the Semi-permeable Membrane:

In the first experiment, 4.3 g ammonium carbamate was loaded into theupper half 15 of the GRC above the membrane 17. The reaction wasconducted for one hour at a temperature of 175°±2° C. under a pressureof anhydrous ammonia of 40±3 MPa.

(e) Reaction Without the Membrane:

In the second experiment, the membrane was omitted and 4.4 g ammoniumcarbamate was loaded into the lower half 16 of the GRC. The reaction wasconducted for one hour at a temperature of 175°±2° C. under a pressureof anhydrous ammonia of 40±3 MPa.

(f) Results:

The conversion of carbamate to urea in the experiment with the membranewas 86.8%. The conversion of carbamate to urea in the experiment withoutthe membrane was 74.1%.

This Example demonstrates conversion of ammonium carbamate to urea inthe CEZ in either of the two ways of use of the invention: either theExternal CEZ as in FIG. 1. or the Internal CEZ as in FIG. 2, with aresidence time of the order of 1 hour under the conditions of theexperiment, with the upper half GRC 15 acting as the CEZ 4 and the lowerhalf GRC 16 acting as the drying zone 8 (see FIG. 1 and FIG. 2). ThisExample further demonstrates a higher conversion in a single pass underthe given conditions by use of the invention than any previouslyreported.

EXAMPLE 2

Experiments were conducted in the same apparatus and following the sameprocedures as in Example 1, with the exception that the membrane usedwas NAFION 423, and the reaction was conducted for one hour at 165°±2°C. (both with and without the membrane).

The conversion of carbamate to urea in the experiment with the membranewas 68.6%. The conversion of carbamate to urea in the experiment withoutthe membrane was 54.8%.

EXAMPLE 3

The same procedures and apparatus as in Example 2 were used in furtherexperiments, with the exception that the reactions with and without themembrane were conducted for two hours at 165°±2° C.

The conversion of carbamate to urea in the experiment with the membranewas 79.1%, while the conversion in the experiment without the membranewas 69.0%.

EXAMPLE 4

The same procedures and apparatus as in Example 2 were used in furtherexperiments, with the exception that the reactions with and without themembrane were conducted for three hours at 165°±2° C.

The conversion of carbamate to urea in the experiment with the membranewas 86.1%, while the conversion in the experiment without the membranewas 71.3%.

EXAMPLE 5

The same procedures and apparatus as in Example 2 were used in furtherexperiments, with the exception that the membrane used was NAFION 324.

The conversion of carbamate to urea in the experiment with the membranewas 76.8%, while the conversion in the experiment without the membranewas 54.8%.

EXAMPLE 6

The same procedures and apparatus as in Example 5 were used in furtherexperiments, with the exception that the reactions with and without themembrane were conducted for six hours at 165°±2° C.

The conversion of carbamate to urea in the experiment with the membranewas 92.8%, while the conversion in the experiment without the membranewas 86.2%.

EXAMPLE 7

The same procedures and apparatus as in Example 1 were used in furtherexperiments, with the exception that the reactions with and without themembrane were conducted under a pressure of anhydrous ammonia of 29±3MPa.

The conversion of carbamate to urea in the experiment with the membranewas 79.7%, while the conversion in the experiment without the membranewas 68.1%.

EXAMPLE 8

The same procedures and apparatus as in example 2 were used in furtherexperiments, with the exception that the membrane used was NAFION 901.

The conversion of carbamate to urea in the experiment with the membranewas 61.6%, while the conversion in the experiment without the membranewas 54.8%.

EXAMPLE 9

The same procedure and apparatus as in Example 2 were used in furtherexperiments, with the exception that the temperature used was 180°±2° C.

The conversion of carbamate to urea in the experiment with the membranewas 86.9%, while the conversion in the experiment without the membranewas 75.7%.

Examples 1 to 9 show that a higher conversion of carbamate to urea isachieved in a given time with the membrane and drying fluid inaccordance with this invention, than without the membrane. In addition,a comparison of Examples 3 and 4, by way of example, shows thatconversion of carbamate to urea in two hours in accordance with theinvention in Example 3 is greater than in three hours without themembrane in Example 4 under otherwise identical conditions. Thisdemonstrates that the rate of conversion is increased in accordance withthe invention.

It will be understood by persons skilled in this art that modificationsor variations may be made to the particular embodiment described indetail herein without departing from the broad principles of theinvention as described above, and that the ambit of the presentinvention extends to encompass all such modifications and variations.

We claim:
 1. A process for the production of urea which comprises thesteps of reacting carbon dioxide and ammonia to form ammonium carbamateand subsequent decomposition of ammonium carbamate to form a reactionmixture comprising urea and water, wherein the reaction mixture iscontacted with one side of a semi-permeable membrane, a drying fluidcapable of removing water and possibly other reaction product(s) fromthe reaction mixture is contacted with the other side of thesemi-permeable membrane, and urea is recovered from the reaction mixtureafter contact with the semi-permeable membrane and optionally also fromthe drying fluid.
 2. A process for the production of urea whichcomprises the steps of reacting carbon dioxide and ammonia to formammonium carbamate and subsequent decomposition of ammonium carbamate toform a reaction mixture comprising urea and water, wherein the reactionmixture is contacted with one side of a semi-permeable membrane, adrying fluid capable of removing water and possibly other reactionproduct(s) from the reaction mixture is contacted with the other side ofthe semi-permeable membrane, and urea is recovered from the reactionmixture after contact with the semi-permeable membrane and optionallyalso-from the drying fluid, and wherein the ammonium carbamate is formedand decomposed to form said reaction mixture comprising urea and waterin a combined synthesis/conversion enhancement zone in contact with saidsemi-permeable membrane.
 3. A process for the production of urea whichcomprises the steps of reacting carbon dioxide and ammonia to formammonium carbamate and subsequent decomposition of ammonium carbamate toform a reaction mixture comprising urea and water, wherein the reactionmixture is contacted with one side of a semi-permeable membrane, adrying fluid capable of removing water and possibly other reactionproduct(s) from the reaction mixture is contacted with the other side ofthe semi-permeable membrane, and urea is recovered from the reactionmixture after contact with the semi-permeable membrane and optionallyalso from the drying fluid, and wherein the ammonium carbamate is formedand decomposed to form said reaction mixture comprising urea and waterin a synthesis zone, and said reaction mixture is then passed to aseparate conversion enhancement zone in contact with said semi-permeablemembrane.
 4. A process according to claim 2 or claim 3 wherein anammonium carbamate-containing recycle mixture is admitted to thesynthesis zone together with said ammonia and carbon dioxide.
 5. Aprocess according to any one of claims 1 2 or 3, wherein saidsemi-permeable membrane has a higher permeability for the transport ofwater, and possibly also urea, than for the other components of saidreaction mixture.
 6. A process according to claim 5, wherein saidsemi-permeable membrane is an optionally reinforced material selectedfrom perfluorocarboxylate and perfluorosulphonate membrane materials, ora combination thereof.
 7. A process according to any one of claims 1 2or 3, wherein said drying fluid is a fluid which can cause water andpossibly other reaction product(s) in said reaction mixture topreferentially migrate across said semi-permeable membrane.
 8. A processaccording to claim 7, wherein said drying fluid is ammonia.
 9. A processaccording to any one of claims 1 2 or 3, wherein said reaction isperformed at a temperature in the range of 140° to 250° C.
 10. A processaccording to any one of claims 1 2 or 3, wherein said reaction isperformed at a pressure in the range of 15 to 50 MPa,.
 11. A processaccording to any one of claims 1 2 or 3, wherein the temperature and/orpressure are adjusted to be substantially the same on both sides of saidsemi-permeable membrane.
 12. A process according to any one of claims 12 or 3, wherein the temperature and/or pressure are different on the twosides of the semi-permeable membrane.
 13. A process according to any oneof claims 1 2 or 3, wherein:a) said reaction mixture is formed at or fedto one side of said semi-permeable membrane; b) said drying fluid is fedto the other side of said semi-permeable membrane; c) an enhancedreaction mixture containing a lower percentage of carbon in the form ofcarbon dioxide and ammonium carbamate than in the said reaction mixtureis removed; and d) urea is recovered from said enhanced reactionmixture, and optionally from said drying fluid.
 14. A process accordingto claim 9 wherein said reaction is performed at a temperature in therange of 160° to 220° C.
 15. A process according to claim 10 whereinsaid reaction is performed at a pressure in the range of 25 to 45 MPa.